Study On The Ultraviolet-Ozone Post-Treatment To Remove The Inherent Impurities In All-Ambient Solution Processed CsPbBr₃ Perovskite Films

Metal-halide perovskite solar cells（PSCs）are a kind of emerging photovoltaic devices.At present,their champion cell’s power conversion efficiency in the laboratory is up to 25.5%,making them strong challengers and competitors of silicon solar cells.In the development of perovskite solar cells,a lot of studies have focused on optimizing the cost-effective and low-temperature solution process to prepare polycrystalline perovskite thin films with high phase purity,good morphology and fewer defects,and thereby fruitful strategies,such as composition engineering,solvent engineering and additive engineering have been developed.In addition,the substances adsorbed on the perovskite surface,such as oxygen and water,will have a great impact on the optoelectronic properties and stability.As the most widely-used preparation method,the solution process,without the ultra-clean environment similar to the vacuum deposition,has unavoidable drawback that can easily be ignored:organic substances may be introduced.These organic substances behave as impurities or defects that deteriorate the performance of final optoelectronic devices.However,the influence of these impurities in the solution-processed perovskite films is less noticed.It has been demonstrated that polycrystalline hybrid perovskite films fabricated in glovebox had considerable inherent impurities containing oxygen and hydrocarbons in both surface and bulk regions despite careful avoidance of exposure to ambient atmosphere.Spontaneous electron transfer occurs between the films and the inherent impurities adsorbed at the interface of its crystal surface,which causes band bending and work function changes,and impedes carrier transport.Among the all-inorganic halide perovskite that have attracted increasing attention,CsPbBr₃ possesses the best moisture and thermal stability.The whole fabrication process of CsPbBr₃ PSCs can be conducted in ambient air,making itself a potential application and an excellent platform for various exploration.It is noteworthy that the chemical composition and optoelectronic properties of the perovskite films will exhibit a significant change when the fabrication process transfer from glovebox to the ambient environment.So,the problem of the above inherent impurities would be much more striking for CsPbBr₃ films fabricated by two-step method in ambient air.Our results show that there are considerable inherent impurities in the surface and bulk region of CsPbBr₃ films,which origin from organic solvents and ambient air.The inherent impurities will affect the chemical environment of cesium,lead and bromin in the CsPbBr₃ crystal.UVO treatment,which can produce high energy photon and strong oxidizing gas simultaneously,is a powerful and widely-used cleaning means to remove organic residual.In the field of PSCs,the UVO treatment can not only clean the substrates,such as transparent conducting oxides and charge transport layer,but also increase surface wettability for better coupling with the subsequently deposited layer,making it an essential step in the process of fabricating PSCs.However,UVO treatment directly on perovskite films to increase their solar cell performance has never been reported.The associated reasons may lie in that the ultraviolet light is generally considered to be detrimental to the stability of the hybrid perovskite solar cells.Meanwhile,the organic components in hybrid perovskites are easily oxidized during UVO treatment.However,given the excellent stability of CsPbBr₃ as well as its all-inorganic components,UVO treatment may be a potential method for removing the inadvertently introduced organic impurities,thereby obtaining high-quality CsPbBr₃-based optoelectronic devices.Our results show that the undesirable inherent impurities attached to the surface and grain boundary can be eliminated effectively without damaging the crystal structure and morphology of the CsPbBr₃ films.After proper UVO post treatment,the device exhibits an improved power conversion efficiency of 7.82%from 7.37%.PL and space charge limited current measurements further confirmed that:for the thin films,the electron trap state density is reduced from 4.05×10¹⁶ cm^-3 to 3.45×10¹⁶ cm^-3,and the average lifetime is prolonged from 1.30 ns to 5.81 ns dramatically,indicating the non-radiative recombination is suppressed.Meanwhile,the voids between the underlying CsPbBr₃films and upper carbon electrode are reduced,exhibiting a much more intimate contact,thereby the charge collection at the interface will be effectively promoted.For the whole device,the assembled PSCs exhibits higher built-in potential,fewer trap-assisted recombination and smoother charge transport.Therefore,the charge carriers can be separated,transported and extracted more effectivelyThis work not only points out the influence of inherent impurities stemming from all-ambient solution process on the optoelectronic properties of CsPbBr₃ films and the performance of the assembled photoelectric device,but also proposes a simple and effective strategy to remove these impurities.Therefore,we hoped that this work will draw researchers’attention to the inherent impurities in optoelectronic materials that are also prepared by solution method in ambient air.In addition,the proposed UVO post-treatment can not only provide ideas and references for cleaning other all-inorganic halide perovskites,but also can be easily integrated into the fabrication process,which will help to advance the commercial process of PSCs.